The present invention is directed towards an improved plastic package for an integrated circuit device, and a method of making such a package.
Integrated circuit device are conventionally enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit device and printed circuit boards. FIG. 1 is a cross sectional view of an inexpensive, conventional molded plastic package 1 known as a plastic dual in line package (xe2x80x9cp-dipxe2x80x9d). The elements of package 10 include an integrated circuit device 12, adhesive material (not shown) to attach integrated circuit device 12 to die pad 17, bond wires 13 that electrically connect bonding pads 14 on integrated circuit device 12 to individual leads 15 of the leadframe, and an adhesive plastic encapsulant material which covers the other components and forms package body 16.
In FIG. 1, molding compound (a.k.a., encapsulant material) is molded above and below device 12, leads 15 and die pad 17 to form package body 16. The peripheral sides 18 of package body 16 are sloped to facilitate release from the mold used to make the package. Leads 15 extend from package body 16. There are a variety of well-known alternative styles for the finishing the leads of such a package, including PLCC and gull wing styles.
Further background information concerning conventional plastic integrated circuit packages and leadframes is contained in chapter 8 of the book Microelectronics Packaging Handbook (1989), which was edited by R. Tummala and E. Rymaszewski, and is published by Van Nostrand Reinhold, 115 Fifth Avenue, New York, N.Y.
The operation of certain types of optical integrated circuit devices, such as erasable programmable read-only memory devices (xe2x80x9cEPROMxe2x80x9d), charge coupled devices (xe2x80x9cCCDxe2x80x9d), and camera-type devices require that light be applied to a light-sensitive cell on a surface of the device. For such devices, standard p-dip packages cannot be used. FIG. 2 is an example of a conventional molded package 20 for an optical integrated circuit device 23.
Package 20 includes some of the same features as package 10 of FIG. 1, so the same reference numbers will be used in FIG. 2. Package 20 of FIG. 2 includes leads 15 and die pad 17, upon which optical integrated circuit device 23 rests. Bond wires 13 extend between individual bonding pads 14 on optical device 23 and an individual lead 15. Encapsulant material is molded above and below the leadframe to form package body 25.
Below leads 15, package 20 of FIG. 2 is the same as package 10 of FIG. 1. Above leads 15, however, package 20 is modified to accommodate optical integrated circuit device 23. In particular, a portion of molded package body 25, namely, support structure 21, is formed on and above portions of leads 15. Support structure 21, which is integral with package body 25 and formed of the same material, surrounds optical device 23 and supports an optically clear plastic or glass cover 27 above optical device 23. Cover 19 transmits light to a light sensitive cell 24 on optical device 23.
Package 20 of FIG. 2 has two significant drawbacks. First, it is difficult to make. Second, it is expensive. Accordingly, there is a need for an inexpensive, easy-to-make package for optical integrated circuit devices.
The present invention includes improved packages for optical integrated circuit devices, and methods of making such packages. In one embodiment, a package within the present invention includes an optical integrated circuit device on a die pad. Metal bonding pads on the optical device are connected by bond wires to leads. The lower half of the package, which supports the die pad and leads, is molded insulative adhesive encapsulant material. The lower and side surfaces of the die pad and leads are covered with encapsulant material, so that the die pad and leads are in an embedded connection with the molded package base. The upper surfaces of the die pad and leads are not covered by the encapsulant material. The side surfaces of the die pad and leads have reentrant portions and asperities that lock the die pad and leads to the molded base. An initially viscous and subsequently hardened insulative adhesive bead surrounds the optical integrated circuit device. The bead extends above the upper surface of the optical device and the bond wires. An optically clear cover, which may be formed of borosilicate glass or optically clear plastic, is adhesively connected to and supported above the device by the bead. For example, the bead is in a press fitted interconnection with the bead. The cover transmits light to a light-sensitive cell on the upper surface of the optical device. The optically clear cover includes locking features which enhance the connection of the cover to the adhesive bead and thus to the package.
A method within the present invention for making such a package includes a first step of providing a leadframe including a die pad and radiating leads. The leadframe is conventional, except that the normally orthogonal side surfaces of the die pad and leads have been modified to include a reentrant portion and asperities that lock the leadframe to the plastic encapsulating material.
Step 2 places the leadframe in a conventional two-pocket mold. The upper pocket of the mold is blanked out by a bar so that encapsulant material does not enter the pocket. Insulative encapsulant material is provided to the lower pocket of the mold. Upon hardening, the encapsulant material forms the lower half of the package. The lower and side surfaces of the die pad and leads of the leadframe are covered by molded encapsulant material. The upper surfaces of the die pad and leads are exposed. The reentrant portion and asperities of the side surfaces of the die pad and leads engage the encapsulate material, and prevent the die pad and leads from being pulled from the molded package base.
Step 3 places and attaches an optical integrated circuit device on the exposed upper surface of the die pad. Step 4 electrically connects individual metal bonding pads on the optical device to individual leads of the leadframe. Step 5 applies a bead of a viscous adhesive material, such as epoxy, around the optical integrated circuit device. The bead covers and adheres to a portion of the exposed upper surface of each of the radiating leads. The location of the bead may vary. For example, the bead may be a distance from the optical integrated circuit device, or alternatively may contact the upper surface of the die pad and the side surfaces of the optical device. In another alternative embodiments, the bead covers the die pad, the side surfaces of the optical device, and a peripheral portion of the upper surface of the optical device.
Step 6 provides an optically clear planar cover. The side surfaces of the cover include locking features, such as an indentation or protrusion, that enhance the connection of the cover to the bead. Alternately, the surface of the optically clear cover that faces the bead includes locking features, such as a groove, located where the bead meets the cover to enhance the connection between the cover and the bead.
Step 7 squarely places the cover on the still-viscous bead so that the cover is centered over the integrated circuit device, and presses the cover into the bead. The bead material supports the cover above the bond wires and optical device. Step 8 hardens the bead. Step 9 is a debar and dejunk step, wherein the dam bar and flash are removed. Step 10 trims and forms the leads. This step removes the tie bar; The leads may be plated before Step 9, or pre-plated leads may be used. The leads may be formed into a variety of conventional styles, such as gull wing, through hole, or PLCC styles.
The packages and methods of the present invention have significant cost advantages over the prior art. These and other advantages will become clear through the following detailed discussion.